Inverse feedback amplifier



y 5,1942 P. WEATHER; 2,281,644

INVERSE FEEDBACK AMPLIFIER Filed Aug. 31, 1939 Suvcntor Patented May 5, 1942 INVERSE FEEDBACK AMPLIFIER Paul Weathers, Audubon, N. J., assignor to Radio. Corporation of America, a corporation of Delaware Application August 31, 1939, Serial No. 292,910

1 Claim.

The present invention relates to inverse feedback amplifiers, and has for its primary object to provide an improved amplifier of the character referred to which is adapted for controlling volume range compression or expansion of signals selectively;

It is-also an object of the present invention to provide. an amplifier having an inverse feedback circuit which includes a current responsive and thermally variable resistance element subject to the output current as a source of, feedback potential, whereby a non-linear response to noise signals and signals having steep wave fronts of short duration is provided, to render such signals ineffective to control the feedbac circuit.

In accordance with the invention, a resistor or other potential drop producing device is connected in series with a thermally variable resistance device, such as a filament or resistance material, responsive to current flow as auariable resistance element, and the series conficte'd'elements are supplied'with signal energy across a suitable amplifier circuit so that the resistance of the filament may be varied in response to signal energy flowing therethrough above a predetermined average amplitude, and dependent upon the current rating of the filament. The junction between the filament and the resistor is utilized as a connection for an inverse feedback circuit to a preceding amplifier stage, and a tap on the signal supply circuit is made to ground so that the feedback voltage derived from the series circuit may be taken either across the resistor or across the filament to provide either volume range compression or volume range expansion of signals transmitted through the amplifier. By utilizing a tungsten filament, the resistance change may be made as high as ten to one, so that the degree of feedback may be made to vary over a relatively wide range. v

It is also a further object of the present inention, to provide an inverse feedback amplifier having a circuit adapted for volume range compression or expansion whereby it may function as an interstage coupling network for controlling the signal output from the inverse feedback amplifier simultaneously with control of the volume range expansion or compression.

The invention will, however, be better understood from the following description when read with reference to the accompanying drawing. and its scope is pointed out n the appended claim.

In the drawing,

Figure 1 is a schematic circuit diagram of an inverse feedback amplifier provided with an inverse feedback circuit for volume range expansion and compression in accordance with the invention,

Figure 2 is a similar schematic circuit diagram of an audio frequency amplifier showing a modification of the inverse feedback circuit of Fig. 1, also embodying the invention, and

Figure 3'is a schematic circuit diagram of a portion of an audio frequency amplifier showing a further modification of the circuit of Fig. 1 in which the inverse feedback circuit and volume control system therefor is utilized as an amplifier interstage coupling network.

Referring to Fig. 1, the audio frequency amplifier shown comprises a first stage amplifier tube 5, resistance coupled through resistance coupling means 6, to one tube 1 of a balanced power output stage 8 of which the other tube is.

shown at 9. The latter tube is coupled to the tube 5 through a suitable phase inverter stage comprising the tube 10, whereby signals applied across the input circuit Hl2 of the first stage 5 are applied to the output stage 1-9 in balanced relation, as is known and well understood.

It will be noted that the input stage 5 is provided with a cathode I 3 in circuit with which is connected a self-biasing resistor 14 without a bypass capacitor, whereas a similar biasing resistor IS, in circuit with the cathode l6 of the inverter stage I0, is provided with an audio frequency bypass capacitor l1. Inverse feedback potentials are applied to the first stage amplifier tube 5 at the cathode and across the resistor l4, through an inverse feedback circuit l8 which is connected with the output circuit of the pushpull or balanced output amplifier 1-4,

In accordance with the invention, the inverse feedback circuit 18 is connected at the junction 20 between a series connected resistor or potential drop producing device 2| and a thermally variable current responsive resistor element such as a tungsten filament lamp 22, the lamp and resistor being arranged in circuit 23 and 24, to be connected across the output circuit 25-46 of the output stage 8.

In the present example, the output circuit is coupled to the lamp and resistor circuit through a coupling transformer 21 which serves to isolate the plate circuit of the output stage from the feedback circuit and from a signal output circuit comprising leads 28 and 29 which may be connected to any suitable signal utilization device (not shown).

The secondary 30' of the transformer is provided with output terminals 3| and 32, the latter being grounded as indicated at 33 and connected to the output lead 29 and to one arm 34 of a double-pole, double-throw switch 35. The terminal 3| is connected to the output lead 28 and to the other arm 36 of the switch. The leads 23 and 24 are connected with terminals 31 and 38 of the switch which are also cross-connected with terminals 39 and 40, respectively, whereby the switch operates as a reversing switch to connect the lamp or feedback circuit with the terminals 3| and 32 of the secondary either with the resistance 2| adjacent the grounded terminal 32 or the lamp adjacent that terminal so that the feedback circuit l8 may be completed to ground 33 either through the lamp or through the resistor as the source of inverse feedback potential.

With this arrangement, it will be seen that with the switch 35 closed and'the amplifier energized, signals applied thereto will appear across the output circuit 2829 and across the resistor circuit 23-24, causing a current to flow through the resistance elements 2| and 22 with a corresponding potential drop therein, a portion of which is applied to the inverse feedback circuit 3-33. The feedback voltage is a portion of the output voltage which depends upon the relative resistance of the two elements 2| and 22.

With the connection as shown, the switch being closed to the contacts 39 and 40, the resistor element 2| is adjacent to the ground connection 33 and provides the source of inverse feedback potential. As the signal strength increases above a predetermined value, the filament of the lamp becomes heated and increases in resistance if of the tungsten type, causing a greater drop of signal potential therethrough than through the resistor 2|, resulting in an inverse feedback through the circuit l8- which is variable with signal strength and with -anon-linear characteristic causing volume expansion since, it will be seen,

the major portion of the potential drop through v tube is shown at and is provided with a signal input circuit 46 and a self-bias resistor 41, across which feedback potentials are applied through ground 48, and an inverse feedback circuit 49 in which is located a controlling capacitor 50 provided with a shunt resistor 5|.

The first stage amplifier is coupled to an output amplifier stage comprising a tube 52, through an impedance. coupling circuit 53, and the output stage in turn is coupled to the series connected controlling resistor 2| and lamp 22 through an output coupling transformer 54, the secondary of which is provided with a plurality of output tap terminals 56, 51 and 58.

The series resistance circuit 2|-22 is provided with a tap at the junction of the resistor 2| and the filament 22 for the inverse feedback lead 49, and the series elements are connected with the terminals er the secondary through supply leads GI and 52. The full output of the secondary 55 is applied to the feedback control elements 2| and 22 in series. In the present example, the ground or retu connection 63 for the feedback circuit is connected through a lead 64 with one of the taps 51 on the transformer secondary 55 to provide a potential balance between the terminal 90 and ground such that for relatively low signal strength the feedback is substantially zero. With this arrangement it will be seen that volume range compression is provided as the lamp filament responds to increased signal strength to increase the resistance relative to that of the resistor 2 I, thereby disturbing the potential balance increasingly as the lamp is heated by the signal current.

The circuit is essentially a bridge circuit in which the feedback connections are made to substantially equal potential points with relatively low signal input. The arrangement may also provide for volume range expansion by reversing the positions of the lamp and resistor, as in the circuit in Fig. 1.

It will be noted that a loudspeaker voice coil 65 is connected between the leads Si and 64, whereby it takes output energ *from a portion of a secondary 65.

The capacitor 50 in the feedback circuit 49 may be of such capacity value that""it forms an appreciable impedance to the flow of signal current to the cathode resistor 41 at relatively low frequencies in the audio frequency range, thereby reducing the degree of feedback at low audio frequencies, and causing signals in the low audio frequency range to be accentuated. Likewise, a capacitor 68 in series with a variable controlling resistor 69 may be connected across the resistor 41 as a shunt path, having a predetermined low impedance to high frequency signals from the feedback circuit, thereby reducing the effect of the inverse feedback in the high audio frequency range to a degree determined by 'the capacitive value of the capacitor and the adjustment of the resistor 89. Consequently the high and low frequency ends of the audio frequency range are not affected by the expansion or compression to as great a degree as is the mid-range, thereby accentuating the low and high frequency response of the amplifier wth increased signal strength.

Referring now to Fig. 3, the series resistor elements 2| and 22 having an inverse feedback connection 60 therebetween and the inverse feedback circuit 49 connected thereto as in the circuit 'of Fig. 2, is provided in the present example as part of an interstage coupling network between an amplifier stage 10 and a following amplifier stage 1|. The network includes a signal circuit comprising two input leads 12 and 13 connected with the secondary 1401' a coupling of a coupling transformer 19 for the stage 1|.

The resistor elements 2| and 22 are connected in series across the secondary or input circuit 12-13 of the network, while the primary or output circuit 16-11 is connected to the tap point 60 between the resistor elements 2| and 22, and with ground 30. The latter is provided with a lead 8| connected with a variable tap 82 forming verse feedback for a predetermined signal strength. For example, the contact 82 may be adjusted to provide at the terminal 82 and at the terminal 60 substantially equal signal potentials with substantially zero or low signal voltage applied to the amplifier stage 10. Upon an inequal amount of volume range expansion. For

example, as the inverse feedback to a preceding amplifier stage is increased, thereby reducing the signal strengthby increasing the feedback, the output signal derived across the circuit IE-ll is increased.

If a signal having a relatively steep wave front or burst of static or other noise voltage is applied to the circuit, the same will cause no appreciable change in the balance of the bridge circuit and, therefore, will not be amplified. Thus, this circuit is effective for damping out undesired signal impulses of that type.

' feedback connection, determines which element As in the circuit of Fig. 2, it will be noted that one side of the feedback circuit is grounded or connected with the common return path, while the opposite side is connected with a tap between the series-connected feedback controlling 'ele-' ments 2| and 22. Furthermore, a potentiometer 82-88 is provided in the present example, the contact 82 of which may be adjusted with respect to the tap 60 to provide a balance of potential at the two tap points for any signal amplitude and substantially zero voltage output to the feedback circuit.

For example, for volume range compression, if the tap 82 is adjusted for a balance at a predetermined gain, it will be seen that, ii. the output exceeds this gain, a decrease in provided by the feedback circuit. I

Likewise, for an expander connection, if the potentiometer is adjusted for maximum feedback at low signal levels, a balance may be reached when the signal output is maximum, thereby pre venting further expander action above a predetermined maximum gain in the amplifier.

With the feedback system shown and described herein,'it will be seen that the inverse feedback is responsive to variations in signal intensity .and is arranged to provide a non-linear dynamic gain control characteristic which may, with one connection, cause volume range compression and which, with another connection, may cause volume range expansion.

The circuit arrangement comprises a filamentary resistor adapted to be heated by the passage of signal current and a series controlling resistor therefor, between which a feedback connection is taken to a preceding amplifier stage. The order of the control elements in'circuit or the location of the elements with respect to the return may provide the feedback potential. Theorder of the elements in circuit may be reversed as seen in the circuit of Fig. 1 to provide volume range expansion or compression of signals and the series control circuit arrangement may be sistor connected across said circuit, a second resistor and a filamentary resistor responsive to current flow .to vary the resistance thereof connected serially across said circuit in parallel with tor is included in said output circuit and in said feedback circuit as a current responsive variable resistance device effective to provide volume range compression and output potential variation gain may be in response to variation in signal amplitude from a level determined by'said potentiometer resistor.

PAUL WEA'I'HERS. 

